A number of devices are known which incorporate radio receivers, which are at least partially analog in nature, and which also incorporate digital circuitry. One such device is the receiver of a watch pager system which is described in U.S. Pat. No. 4,713,808, Dec. 15, 1987, to Gaskill et al. Digital circuitry in such a device may operate at a known frequency, and generally includes more than one clock, for timing various portions of the circuitry. The radio receiver may operate in a known frequency range, such as the commercial FM band used in the United States, which operates between 88 MHz and 108 MHz.
In the case of the watch pager system described in the '808 reference, communication signals are transmitted from conventional FM broadcasting facilities on FM side bands and are received by the watch receiver. The receiver may operate at a frequency in the high 80's or low 90 MHz range. The digital portion of the receiver may operate at a nominal frequency of 10 MHz. Such digital circuitry usually radiates harmonics at 80 and 90 MHz, which interferes with the receiver function.
RF interference, or crosstalk, between analog and digital signals is common in large scale integrated circuits wherein analog/digital applications are both present and are distributed in the system. Particularly in cases where analog and digital portions of the circuit are in close proximity to one another, and wherein the digital signals radiate at harmonic-producing frequencies, the harmonics may significantly impact the quality of the analog signal which is received by the receiver and degrade the resultant receiver function.
Shielding is one technique which may be used in an attempt to prevent the digital origin harmonics from interfering with the analog radio receiver. Shielding requires that the digital portion of the circuitry be encapsulated, or "boxed-off", in an attempt to prevent the harmonics emanating from the digital circuitry interfering with the analog portion of the receiver.
A modified form of shielding provides a top cover, which merely extends over the top of the digital portion of the circuitry. Another type of shielding is a full encapsulation of the digital circuitry, completely enclosing it within a shield. In devices such as the wristwatch pager receiver described in the '808 reference, such shielding is not practical for at least two reasons. First, complete encapsulation of the digital circuitry is not practical because of the small size of the circuit. Second, in the case of long connector runs, wherein the digital portion of the circuit may actually leave an integrated circuit and traverse a circuit board, shielding is extremely difficult, if not impossible.
Another interference avoiding technique requires shifting the frequency of the clocks that drive the digital circuitry each time the radio receiver is tuned to a new frequency so that the harmonics generated by the digital circuitry do not fall within the receiver's band of interest. The preceding technique requires additional circuitry which senses the frequency of the initial signal reaching the receiver and operates to shift the frequency of the digital circuit to avoid generating an interfering signal. This technique again requires additional circuitry and hence, additional space and cost.
Another technique for reducing the generation of harmonic interference is to operate an integrated circuit containing the digital circuitry at a low voltage, for instance, from 1 to 1.5 V instead of the normal 5 volt integrated circuit operating voltage. In order to operate an integrated circuit at the lower voltages, an application specific integrated (ASIC) must be designed. Such a circuit may be a totally custom-design IC, wherein each device in the circuit is individually designed to meet the requirements for the particular device. Such a circuit generally achieves a higher performance standard, using lower power and has a smaller die size, however, such custom designs require a great deal of design time and increase the chance that errors in circuit design will occur.
An ASIC may also be designed using a standard cell library, wherein the various devices in the circuit are chosen from a set of alternatives available in a standard cell library. Such ICs may be designed more quickly and with less probability of error, however, the speed, power and die size of such ICs are generally not optimized. In some situations, standard cells may be used for parts of the circuitry and custom designs may be used for other parts where performance is critical.
An IC may be designed such that the digital circuits which operate at frequencies having harmonics within the receiver's band of interest have a width-to-length (W/L) ratio as low as possible. Low W/L ratio circuits operate at minimal currents and have pulses which are substantially rounded in nature, which generate relatively few harmonics. However, such a fully custom integrated circuit is an exceptionally expensive and time consuming to design.